Field of the Invention and Related Art Statement
The present invention relates to an apparatus for reproducing information from a magneto-optic record medium in which the information has been recorded by magnetizing a recording layer thereof in a direction perpendicular to a plane of the recording layer.
Hitherto, several kinds of apparatuses for reading out information from magneto-optic information record medium have been suggested. For instance, in Japanese Laid Open Patent Publication No. 63-184936, a known apparatus for reproducing information from a magneto-optic record medium is disclosed and the construction thereof is shown in FIG. 1. In this known apparatus, a laser beam is generated by a semiconductor laser 1 and is projected on a magneto-optic information record medium 2 via a collimater lens 3, a beam splitter 4 and an objective lens 5. A laser beam reflected by the record medium 2 is made incident upon a polarization beam splitter 6 via the objective lens 5, the beam splitter 4 and a half wavelength plate 7. The light beam is separated into a transmission light beam and a reflection light beam by the polarization beam splitter 6. And the transmission and reflection light beams are detected by first and second photodetectors 8a and 8b, respectively, and outputs of the first and second photo detectors 8a and 8b are supplied to a differential amplifier 9 to obtain a signal representing the information recorded in the optomagnetic record medium 2 by detecting a difference between the outputs of the photodetectors 8a and 8b.
In this known apparatus, when a linearly polarized light beam, whose polarizing direction is represented by an x axis in FIG. 2A, is made incident upon the record medium 2, a plane of polarization of the reflection light beam reflected by the record medium 2 is rotated by .theta..sub.K by the Kerr effect. In principle, the reflection light beam reflected by the record medium 2 is also linearly polarized light beam. The direction of the linearly polarized light beam reflected by the record medium 2 is represented by a line A in FIG. 2A.
However, the record medium 2 is generally formed to have a multiple layer construction as shown in FIG. 3 in order to increase the recording ability thereof. The record medium 2 is constructed such that a dielectric layer 2c is provided between a transparent substrate 2a and a magnetic film 2b which serves as an information recording layer. When the incident linearly polarized light beam is made incident upon the record medium 2 having such multi-layer construction, there is generated a phase difference between a component of the light beam reflected by the record medium in the polarization direction of the incident linearly polarized light beam and a component in a direction perpendicular to the polarization direction of the incident linearly polarized light due to a characteristic of the magnetic film 2b per se and optical characteristics of the other layers constituting the record medium 2. Therefore, the linearly polarized incident light beam is often converted into an elliptically polarized light beam as shown in FIG. 2B when being reflected by the record medium 2. In this case, the rotational angle of the polarized plane of the incident light beam rotated by the Kerr effect is determined by an angle between the major axis of the elliptically polarized light beam and the direction of the incident linearly polarized light beam, so that the rotational angle .theta..sub.K ' is substantially decreased as shown in FIG. 2B in comparison with the angle .theta..sub.K of FIG. 2A. And therefore, a strength of the signal (C/N ratio) detected by the differential amplifier 9 is decreased. When the phase difference introduced by the multi-layer construction of the record medium is represented by .delta., the decrease of the C/N ratio is proportional to cos .delta.. The decrease of C/N ratio is caused by similar phase differences of the optical elements provided in the information reproducing apparatus such as the beam splitter and mirror (not shown in FIG. 1).
In order to compensate for the above mentioned phase differences, there has been suggested to arrange a Babinet compensator 10 between the beam splitter 4 and the half wavelength plate 7 as shown in FIG. 4. The Babinet compensator 10 comprises a pair of wedge-shaped dextrorotatory crystals 10a, 10b and a laevorotatory crystal 10c which is adhered to one of the wedge-shaped dextrorotatory crystals 10a such that the optical axis of the crystal 10c is perpendicular to that of the crystals 10a and 10b. It should be noted that the optical axis of the dextrorotatory crystal 10a is represented by a mark and the optical axis of the laevorotatory crystal by a mark .sym. in FIG. 4. The phase difference caused by the opto-magnetic record medium 2 and/or the optical elements constituting the optical system of the information reproducing apparatus can be compensated for by moving the wedge-shaped dextrorotatory crystal 10b by means of a micrometer. Thus, it is possible to convert the elliptically polarized light beam into a linearly polarized light beam. Therefore, when the Babinet compensator 10 is arranged between the beam splitter 4 and the half wavelength plate 7, the Kerr rotation angle does not decreased so much that the signal having a high C/N ratio can be obtained. However, the Babinet compensator 11 is so expensive and has very complexed construction as stated above. Further, in order to compensate for the phase differences accurately, it is necessary to adjust the compensator in a precise manner and this requires time consuming and cumbersome operation. Therefore, it is practically difficult to use the Babinet compensator.